1. Field of the Invention
This invention relates to an electronic sewing machine provided with an improved control system for controlling a DC motor employed for driving upper and lower shafts of the sewing machine. The control system will automatically detect an overloaded condition in the sewing machine.
2. Description of the Prior Art
It has been recognized necessary that a loaded torque applied to an electronic sewing machine be within a predetermined reference range. An example of the prior art electronic circuits mounted within the sewing machine for self-diagnosis of the overloaded condition is shown in FIG. 7. In this circuit, a volume VR1 of a controller is connected to a power source Vcc whereby a voltage value which is controlled and changed depending upon an operating degree of the controller will be applied via a filter consisting of a resistor R1 and a capacitor C1 to an analog-digital converting terminal A/D1 of a one-tip contral processing unit (hereinlater referred to as IC2). A pulse-width-modulation (PWM) signal which is corresponding to the voltage value thus determined will be outputted from IC2 for the control of a DC motor M which drives the sewing machine. To a reset terminal of IC2 is connected an integrated circuit (IC1) for detecting every rise and fall of the power source Vcc to initialize IC2. The motor M is connected between another power source Vb and a drain of a power MOSFET (FET1). In parallel with the motor M is connected a free wheel diode D1.
The resistor R5 is connected to a source of FET1 for detecting the current value inputted to the motor M. A signal representing a voltage which is in turn corresponding to the current value detected by the resistor R5 is supplied therefrom, via a filter consisting of a resistor R6 and a capacitor C3, to an analog-digital converting terminal A/D3 of IC2. There is provided a diode D2 for preventing an excessive voltage from being inputted to the terminal A/D3. The PWM signal outputted from IC2 is supplied via a gate IC3 and a gate protecting resistor R4 to a gate of FET1.
A sensor 10 is provided for detecting a rotating speed of the motor and supplying a detection signal toward a port P1 of IC2. The sensor 10 may comprise, as known, a disc plate provided with a slit and secured to the sewing machine upper shaft or a main drive shaft and a photo-interrupter device cooperating with the disc to detect every rotation of the shaft, thereby detecting the rotating speed of the motor M.
There are also provided a potential divider consisting of two resistors R2 and R3 for detecting a voltage supplied from the motor drive power source Vb. A divided voltage obtained at a connecting point between these resistors R2 and R3 should be set to a value which does not exceed 5 V, a voltage value supplied from the control power source Vcc, and which is supplied to the terminal A/D2 of IC2. A capacitor C2 serves as a filter for the divided voltage value.
With the prior art system shown in FIG. 7, the current value inputted to the sewing machine motor M can be detected by the resistor R5. However, when a current flow is supplied to the resistor R5 which is connected in series with the motor M, a source potential is increased so that a gate voltage is relatively decreased, thereby making it impossible to drive FET1. To cope with this problem, it has been necessary to provide another power source Va having a voltage value, e.g. 12 V which is substantially higher than 5 V of the control power source Vcc, to be supplied to the gate of FET1. An additional problem that the prior art has incurred is that the current detecting resistor R5 should be of a considerably great degree of capacity. Supposing the resistor R5 will detects a voltage value of about 1 V when the sewing machine is driving with a rated current of the DC motor which is determined to be about 1 A, the resistor R5 will have a resistance of 1.OMEGA.. However, under the locked condition of the sewing machine, the resistor R5 would receive an overcurrent of at least 10 A, resulting in heat generation of (10 A).sup.2 .times.(1.OMEGA.)=100 W or more. In an actual system, there is provided a safe guard means for cutting off the electricity supply to the motor in a while responsive to detection of the locked condition of the sewing machine so that a capacity of the resistor R5 may be decreased to the order to a fraction of 100 W. Even if so, the current detecting resistor R5 has still been required to have a considerable capacity.
Another prior art for detection of the overloaded condition of the sewing machine DC motor will involve substantially constant detection of the rotating speed of the motor. When the rotating speed of the motor should be lowered below a predetermined limit, which should be changed in correspondence with the preset rotating speed which may be selectively determined as desired by operation of the controller, a control means is actuated to automatically interrupt power supply to the motor. For example, after the actual rotating speed S has reached the preset rotating speed S1, it would be lowered as from a time T1 due to a load involved, as shown in FIG. 5. In cases shown by curves A to C, the decreased rotating speed is still higher than a predetermined limit S2, the control operation will not be effected so that the motor continues to rotate under the said loaded condition. In a case shown by a curve D, however, the actual rotating speed S is decreased below the limit S2, in which case the motor will be prevented from further power supply. Once the power supply to the motor is interrupted, it will be necessary to again operate the controller or depress a start button for re-driving the motor.
In accordance with the prior art discussed above, there would be a risk of the motor and a motor drive circuit connected thereto being overheated by repeated drive/stop operation, which may result in machine troubles. In some case, a machine component would be destroyed by fire. Moreover, the overloaded condition shown by the curves A to C in FIG. 5, for example, could not be detected by the control system of this prior art. If the motor is driving under such an overloaded condition for a long period of time, there would be a cause of various machine troubles, as well.
It could be considered that a temperature sensor be arranged for detecting a temperature rise of the sewing machine motor during continuous driving thereof. In practice, the temperature sensor could not be mounted within the motor but is attached to the outer periphery. However, as clearly demonstrated by the graph in FIG. 6, the temperature sensor attached to the outside of the motor could not detect an actual temperature rise occurring inside the motor.